Flat-panel light-emitting devices, where the light is generated inside luminescent media, are known in the art. Examples of such devices are, for example, electroluminescent flat panel displays such as TFEL or OLED displays. Another example is flat OLED lamp, as described in U.S. Pat. No. 6,936,964 entitled “OLED Lamp.” As shown in FIG. 1, the generated light 1 exits at least through one surface 2 of device 10 into air either, through (semi)transparent electrode 3 and (semi)transparent substrate 4 and/or lid 5.
Typically, the generated light radiates from the (electro)-luminescent medium equally in all directions; these are known as Lambertian light sources (i.e., the angular light distribution follows Lambert's cosine law (FIG. 2)). However, due to the fact that the medium has refractive index greater than 1 (n>1), the amount of light that can leave the exit surface(s) is governed by the condition known as total internal reflection (TIR), i.e., only light rays with angle of incidence smaller than critical angle will be able to exit the said surface (i.e., be extracted from) the device. The ratio of amount of light extracted to amount of light generated is called outcoupling efficiency into air ηout and is commonly expressed in percentages (%). The outcoupling efficiency through one surface (into one hemisphere) is given by Equation (1) below:ηout=½(1−cos θc)≈¼n2  Equation (1)whereθc is the critical angle; andn is the refractive index of the medium.
In common practice, the outcoupling efficiency is described by formula set forth in Equation (2) below:ηout≈½n2  Equation (2)as the light radiated in the other hemisphere is also harnessed for practical purposes (by employing reflective electrode, for example).
In a typical configuration, where the luminescent medium is a thin film of sub-wavelength thickness surrounded by glass with n≈1.5, the above formula in Equation (2) yields ηout≈22%. Thus only about 22% of light generated inside the medium is used for illumination or reaches the viewer to convey the displayed information. This inefficient light extraction has a negative impact on the overall efficiency of such electro-optical devices.
Several approaches have been suggested to improve the light outcoupling efficiency from flat panel displays or lamps. Examples of prismatic structures (or variations like lenslet arrays or cone arrays or similar structures) can be found in open literature, for example, in U.S. Pat. No. 7,011,420 entitled “Planar Directed Light Source.” Another approach uses optical interference to produce microcavity effect for each of elemental color (red, green and blue), as described in U.S. Pat. No. 6,917,159 entitled “Microcavity OLED Device.” Another specific application to OLED display is described in U.S. Patent Application Publ. No. 2005/0266218 entitled “Aperiodic Dielectric Multilayer Stack,” wherein the authors inserted aperiodic stacks of dielectric thin films into the structure of the OLED display.
While the above-described approaches mitigate the effects of the TIR to certain degree, they rarely improve the outcoupling efficiency into air by more than factor of two. In addition, the above-mentioned solutions either degrade the quality of images on displays (prismatic structures) or are viewing angle dependent (microcavity) or wavelength dependent (aperiodic stacks of dielectric thin films). Thus, there is a need in the art for a solution that will strongly mitigate, or fully remove, the limitation of TIR on outcoupling efficiency without the undesirable effects on image quality.